There may not be any comparison between Angel after 100 years of living with a soul and Spike having done the same thing for only a few months at best for you, but apparently there was for Buffy, and I do agree that trusting him that soon was a risk, but either Buffy didn't see that difference, or believed that she would be able to handle whatever happened. I don't necessarily agree, I'm only pointing out what she probably thought, if she even thought about it at all.
After Buffy found out that Spike was killing again, she chained him in the basement, which is where he was when the Bringers took him. Afterward, Spike chained himself up down there unless Buffy was with him.
And though this is getting to the point of defending Spike, which I refuse to do anymore, I will point out that Spike was able to resist the Fiest Evil's directions once he knew that something was trying to control him. Prior to that, he had no memory of anything that he did when the trigger was activated, or that anyhing like the trigger even existed.